Method and apparatus for estimating applied wheel torque in a motor vehicle

ABSTRACT

A vehicle traction control system is controlled in part by a signal value indicative of estimated wheel torque. The estimated wheel torque value is produced within the vehicle&#39;s electronic engine control (EEC) module by summing a first value which indicated the estimated torque attributable to engine combustion and a second value which is proportional to engine acceleration/deceleration which indicates the amount of torque attributable to the inertial movement of engine and drive train masses. The second value is modified by a third value based on the speed ratio across the transmission. Before summing the two signal components, the signal which indicates combustion torque is preferably delayed with respect to the signal indicating inertial torque by a delay interval whose duration varies with engine speed to take into account the delay between intake fuel rate changes and combustion forces as well as delays attributable to the timing of the calculations themselves.

FIELD OF THE INVENTION

This invention relates to electronic vehicle control systems and moreparticularly, although in its broader aspects not exclusively, totraction control systems and the like.

BACKGROUND OF THE INVENTION

Vehicle traction control and traction assist systems commonly employ twocontrol modules: (1) a anti-skid brake system (ABS) module whichcontrols the application of torque to the wheels from both the engineand the braking system, and (2) an electronic engine control (EEC)module which responds to commands from a variety of sources, includingthe ABS module, to control the amount of power produced by the engine.To achieve traction control, the ABS module receives a value from theEEC module indicating the estimated wheel torque, which the ABS modulethen uses to form and evaluate control commands sent to the EEC moduleand the braking system.

In prior systems, the EEC module produces the estimated wheel torquevalue by first calculating actual engine torque based on the currentengine operating conditions, and then employs the resulting enginetorque value in combination with vehicle status information (the currentgear ratio and losses caused by other power consumers such as the airconditioner, etc.)

Comparisons of the actual measured wheel torque with the estimated wheeltorque values produced by conventional methods has shown that thesepredicted torque values are prone to error because the mechanism forgenerating the estimated value does not take all inertial forces andother effects into account.

In co-pending application Ser. No. 08/524,274, assigned to the assigneeof the present invention, which is incorporated herein by reference inits entirety, provides a means for calculating driveline inertia toimprove traction control function. The present invention improves onthese calculations by separating the inertia of the torque converter,shuffle mode dynamics and differential torque differences.

SUMMARY OF THE INVENTION

The present invention takes the form of a vehicle control system, suchas a traction control system, which incorporates an improved method forestimating the amount of torque applied to a vehicle's wheels by theengine, transmission and drive train.

In accordance with a first feature of the invention, means are employedfor generating a first value which indicates the amount of torquegenerated by combustion within the engine and for generating a secondvalue which indicates the amount of torque attributable to the inertiaof the moving engine. The second value is modified by a third valuebased on the speed ratio across the torque converter. Means are thenemployed for forming an estimated wheel torque value by summing thefirst and modified second values, thereby accounting for inertial forceswhen the engine and power train is accelerating or decelerating.

In accordance with a second feature of the invention, means are employedfor delaying the first value representative of torque attributable tocombustion with respect to the second value representative of inertialtorque by a variable time delay interval prior to combining those firstand second values.

As contemplated by the foregoing second feature of the invention, thevariable time delay interval has a duration which takes into account theengine's inherent combustion delay; that is, the delay between thechanges in the intake air and fuel rates and consequent changes inengine torque attributable to those intake changes, and further takesinto account delays attributable to the signal processing required togenerate the torque estimates. The interval by which the combustiontorque value is delayed with respect to the inertial torque value isaccordingly determined in response to current engine speed, withincreasing engine rpm resulting in a corresponding decrease in the delayinterval.

Another aspect of the present invention includes calculation of asurface coefficient of friction when wheel slippage is present or whenthe detected coefficient of friction increases. The improved torquecalculations provided herein provide for better friction coefficientdetermination.

The methods and apparatus contemplated by the present invention arepreferably implemented using a conventional electronic engine controlmodule which employs a micro controller to process input signal valuesto produce the desired applied wheel torque values. These torqueestimates are calculated at repeated intervals within a timed"background" process repeatedly initiated at a predetermined timedintervals. In accordance with this aspect of the invention, the desireddelay interval is advantageously formed by an integral number ofbackground loop intervals. Accordingly, to introduce a delaycorresponding to the combustion delay while also taking signalprocessing delays into account, a first (engine torque) value producedduring a selected prior background loop process is combined with thesecond (inertial torque) value produced during the current backgroundloop process.

The more accurate applied wheel torque values produced in accordancewith the present invention improve the performance of the tractioncontrol systems and the like by sharing instrumentalities, including themicro controller, sensors, and related devices which are used in theconventional EEC module, thus providing improved performance withoutadding significant additional costs or significantly increasing thecomputational burden placed on the EEC micro controller.

These and other features and advantages of the present invention may bebetter understood by considering the following detailed description of apreferred embodiment of the invention.

During the course of this description, frequent reference will be madeto the attached drawing.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a schematic block diagram of a preferred vehicle tractioncontrol system which embodies the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The illustrative control system utilizing the invention depicted in theFIGURE is used with an internal combustion engine indicated generally at10. The engine's crankshaft 12 applies torque to a driven wheel 14through a conventional drive train illustrated at 16. The amount oftorque applied to the drive wheel 14 by the engine 10 through the drivetrain 16 is controlled by an electronic engine control (EEC) module 20which utilizes the invention as described below.

In addition to the torque applied to the wheel 14 by the drive train 16,braking torques are applied by a brake system 22 which is controlled bya conventional anti-skid brake system (ABS) module 24. In addition tocontrolling the brake system 22, the ABS module also sends commands tothe EEC module 20 via a communication pathway 26 to indicate the amountby which applied wheel torques should be reduced such that theapplication of excessive engine torque via the drive train 16 does notcause a loss of traction. In order to determine the magnitude of thedesired torque reduction, the ABS module 24 utilizes a value suppliedfrom the EEC module 20, as indicated at 25. The value 25 is generated bythe EEC module 20 in the manner to be described to provide an indicationof the estimated magnitude of the torque applied to wheel 14 via thedrive train 16.

Conventional EEC modules have heretofore produced the required estimatedwheel torque value based on the magnitude of air and fuel beingdelivered to the engine, taking into account various vehicle statusindications, including the current transmission gear ratio and theextent to which other systems, such as the vehicle's air conditioner,alternator, etc. impose a load on the engine which is reflected as aloss of wheel torque.

Although these prior techniques have produced workable estimates ofwheel torque, they are subject to inaccuracy, particularly duringperiods of significant acceleration and deceleration. Inertial torquesproduced by the drivetrain, including the moving engine and particularlythe drive train masses, are not adequately separated and taken intoaccount in conventional systems. During acceleration, the resultingoverestimation of applied engine torque can lead to excessivelyaggressive reductions in engine power, or to the application ofexcessive braking torques, resulting in poorer driveability. Wheninertial effects are ignored during deceleration, the estimate ofapplied wheel torque is too low, the need for reduced engine power isunderestimated, and the effectiveness of the traction control systemduring engine braking is adversely affected.

In order to form an accurate estimate of applied wheel torque, it isalso important to take into account the relative timing of the effectsbeing taken into account, as well as the signal processing delays whichoccur as the estimates are generated.

An indicated change in the rate at which air and fuel are supplied tothe intake of an engine is not immediately manifested as a correspondingchange in the speed of the engine and the drive train masses coupled tothe crankshaft. A "combustion delay" typically on the order of 270° to360° of crankshaft rotation must occur before changes at the intakeresult in changes in combustion that effect the engine's speed.Depending on the fuel injection scheme employed, this delay could be asmuch as 720°, in the case of bank-to-bank fuel injection for example. Asa result, estimated engine torque values which are determined fromintake flow rates should be delayed by an interval representing thecombustion delay before being combined with the inertial torque.

Moreover, when the estimate is produced by the EEC module, the signalprocessing occurs at timed intervals during "background loop" periods.As a consequence, there is a computational delay between engine torqueestimates which should be taken into account when the estimated wheeltorque is determined in order to achieve the greatest accuracy.

The preferred embodiment of the invention shown illustrated in theFIGURE is preferably implemented by using the processing capabilities ofthe micro controller and the associated vehicle and engine statussensors which are already used for conventional fuel control processingwithin a conventional EEC module. As depicted in the FIGURE, the EECcontrol system 20 utilizes a conventional air and fuel control mechanism31, and possibly a spark timing control mechanism (not shown), torespond to the torque reduction commands from the ABS module 24 via asignal pathway 26. The fuel and air control mechanism 31 operates a"drive by wire" throttle valve indicated at 33 to control intake airflow, and further provides control signals to a fuel rate control unit32.

The fuel rate control unit 32 controls the fuel delivery rate suppliedby fuel injector(s) indicated at 34 in response to several inputsincluding: a measured intake air flow from an air flow indicatorillustrated at 37; a supplied measurement of the oxygen level in theengine exhaust as indicated at 39; and from the current engine speedprovided by an RPM indicator 40. The fuel rate control unit 32 maintainsthe air/fuel mixture at or near stoichiometry to minimize undesiredcombustion.

The EEC module 20 further includes a conventional mechanism 44 forgenerating an output value indicative of the estimated engine torquebased on the quantity of combustibles being supplied at the engineintake. The EEC module processing that produces the output value whichindicates "combustion torque" may usefully take into account a number ofeffects, including torque converter characteristics specified by torqueconversion curve values which are produced as a function of enginespeed. The combustion torque values calculated at 44 may alsoadvantageously take into account higher frequency (3-10 hertz) torquevariations resulting from drive train "shuffle" mode dynamics, asdiscussed further below.

As contemplated by the present invention, the resulting enginecombustion torque estimate thus derived is then delayed by a time delaymechanism 46 for a variable delay interval and is then combined with avalue supplied via line 50 representing the combined torquesattributable to engine losses and the inertia of the engine, torqueconverter, differential, and related systems.

A delay unit 46 as shown in the FIGURE delays the value of estimatedengine torque determined at 44 by an integral number of "background looptimes" depending on the engine's actual rpm. Conventional EEC controlsystems perform the processing (seen at 44) necessary to generate theestimated engine torque in a "background loop" which is initiated atpredetermined clocked intervals established by the micro controller'sinterrupt rate. These calculations are thus performed to generate a newengine torque estimate approximately once every 50 ms (20 times persecond), referred to as delayed engine torque. As a consequence, sincethe engine torque estimates are only formed at intervals, the delay tobe imposed on the engine torque estimate before it is adjusted toreflect the inertial effects and engine loading, should also be inintegral "background loop time" units.

Based on measured comparisons of actual wheel torque with the torqueestimated using the present invention, it has been determined that animprovement in accuracy can be obtained, given a background loop time ofapproximately 50 ms., by delaying the engine torque estimate withrespect to the inertial torque estimate by two background loop timedelays (100 ms.) when the engine speed is less than approximately 1,500rpm; by one loop interval while engine speed is between approximately1,500 and 3,000 rpm; and by introducing no delay at all when the enginespeed exceeds approximately 3,000 rpm.

Accordingly, the current and last two engine torque estimates arestored. The delayed engine torque is determined using these threevalues. When engine speed is below 1500 RPM, the oldest of the threeengine torque estimates is used for delayed engine torque. When enginespeed is between 1500 and 3000 RPM, the first prior torque estimate isused for delayed engine torque. When speed exceeds 3000 RPM, the currenttorque estimate is used for delayed engine torque.

The component of wheel torque which is attributable to engine inertialeffects is determined by first determining the rate-of-change in engineRPM at 51 and then multiplying the resulting acceleration/decelerationvalue times a constant which represents the inertial mass of the engineto determine an engine inertial torque. The predetermined engine inertiaconstant is calculated or measured to reflect the mass of the movingengine. The multiplication of engine acceleration times the moment ofinertia value is seen at 52 in the FIGURE and produces a value at 53indicating the estimated inertial torque attributable to the engineinertia. Net instantaneous engine torque may be calculated bysubtracting the engine inertial torque from the calculated delayedengine torque.

In a preferred embodiment, the engine inertia torque is bounded by thedelay torque, such that if the absolute value of the engine torqueexceeds the absolute value of the delayed torque, then the value of theengine inertia torque is limited to the value of the delay torque(positive or negative). The bounded engine inertia torque is thensubtracted from the delay torque to yield the net instantaneous enginetorque. This bounding reduces errors particularly when some of theengine cylinders are not being fueled to reduce engine torque.

The present invention accounts for the inertia of the torque converteras shown in the FIGURE at 60, when a torque converter is present (in avehicle so equipped). First, a speed ratio (SR) is calculated bymultiplying the average driven wheel speed (Nwd) by the gear ratio (GR)divided by the engine speed (Ne). The gear ratio (GR) is the product ofthe transmission gear ratio and the final drive ratio.

Secondly, a torque ratio (TR) is determined from a lookup table todetermine the torque amplification across the torque converter. Thetorque ratio (TR) is interpolated for the particular speed ratio acrossthe transmission 15. When no torque converter is present, such as in avehicle with a manual transmission, the torque ratio=1. The torque ratio(TR) is used below at 61 when calculating the torque converter anddriveline inertial effects. The sample torque ratio table provided belowincludes a speed ratio column and a torque ratio column for a particulartorque converter setup:

    ______________________________________                                        Speed Ratio (SR)                                                                            Torque Ratio (TR)                                               ______________________________________                                        0             2.4                                                             0.4           1.73                                                            0.83          1                                                               1             1                                                               ______________________________________                                    

The effective driveline inertial torque is calculated by multiplying theeffective driveline inertia (J_(d)) by the driven wheel acceleration(dNwd/dt). The driveline rotational inertia J_(d) is a constantdetermined from the vehicle characteristics, similar to thedetermination for the engine, as described above.

The transmission torque is calculated at 61 by first multiplying the netinstantaneous engine torque and the torque ratio (TR); subtracting theproduct of the effective driveline inertia torque and the gear ratio(J_(d))*(dNwd/dt)*(GR); and finally, that result is multiplied by onehalf of the gear ratio to produce the transmission torque (Tt).

A shaft torque (Ts) is then determined at 62 by passing the transmissiontorque (Tt) through a first-order low-pass filter chosen appropriatelyto reflect the driveline compliance (G(s)). In a preferred embodiment,the filter is 10 Hz. In an alternative embodiment, the shuffle modefilter includes second-order (oscillatory) dynamics. Determination ofsuch a filter is described in further detail in U.S. Pat. 5,452,207,assigned to the assignee of this invention, which is incorporated hereinby reference in its entirety. Shuffle mode dynamics are described indetail in a paper published by the Franklin Institute entitled "BondGraph Modeling of Automotive Power Trains" by Hrovat, D and Tobler, W.,1991, at p. 621, which is incorporated herein by reference.

Although not illustrated here, in a preferred embodiment, the torqueloss due to friction in the differential is also compensated for afterthe shaft torque estimate is completed. The differential torquedifference is a function of the difference in rotational speed betweenthe right and left halfshafts. The resultant spinning shaft torque isproduced by subtracting the differential torque from the shaft torquedescribed above. The differential loss is a monatomically increasingfunction, and in some instances has been calculated as a simpleproportional function. One skilled in the art preferably determines sucha loss by mapping the differential speed during dynamometer testing anddevelops a function to compensate therefor.

The estimated inertial torque value at 53 is combined at 54 with torquelosses from other sources as indicated by operating status inputs fromthe source 55 which indicate factors as the additional load on theengine imposed when the air conditioner is operating and when thealternator is drawing power.

The values of the inertial effect are combined at 54 to form a currenttorque adjustment value on line 50, which is then combined additivelywith the (possibly delayed) estimated engine torque value supplied fromthe delay unit 46. The combined signal at 25 is supplied to the ABSmodule 24 and/or to other vehicle control system functions that utilizethe value of estimated wheel torque.

The improved shaft torque estimation enables a more accurate calculationof the coefficient of friction. Once the shaft torque is determined 62,the wheel torque is calculated at 63 by subtracting the wheel inertialtorque from the shaft torque. The wheel inertial torque is the productof the wheel rotational inertia (J_(w)) and the average acceleration ofthe driven wheels (dNwd/dt). The wheel rotational inertia is a constantdetermined from the vehicle characteristics.

Wheel slippage is calculated at 64 by dividing the difference betweenthe speeds of the driven wheels and the nondriven wheels by the speed ofthe driven wheels. A new estimated coefficient of friction is nextcalculated at 65 by dividing the wheel torque by the product of thenormal wheel force and the wheel dynamic radius. If the new estimatedcoefficient of friction is greater than the previously calculatedcoefficient of friction (the present coefficient of friction) or if thewheel slippage exceeds a calibratible amount, which in a preferredembodiment is 0.15, then the new coefficient of friction is establishedas the present coefficient of friction.

It is to be understood that the embodiment of the invention describedabove is merely illustrative on one application of the principles of theinvention. Numerous modifications may be made to the methods andapparatus described without departing from the true spirit and scope ofthe invention.

We claim:
 1. A traction control system for a vehicle comprising:aninternal combustion engine connected to a transmission, the engineequipped with a fuel rate controller responsive to a first controlsignal for applying a variable amount of drive torque to at least onedriven wheel through a drive train, a brake system for applying brakingtorques to said drive wheels in response to a second control signal, ananti-skid control module connected to supply said first signal to saidfuel rate controller and further connected to supply said second controlsignal to said brake system, said antiskid control module beingresponsive to a third control signal having a value indicating theestimated magnitude of said drive torque, means coupled to said fuelrate controller for generating a first value indicative of the amount ofwheel torque generated by said internal combustion engine, means coupledto said engine for generating a second value proportional to the rate ofchange of the operating speed of said engine and indicative of theamount of wheel torque produced by the inertial movement of said engine;means coupled to said transmission for generating a third valueindicating a torque ratio as a function of the speed ratio of thetransmission indicative of the amount of wheel torque produced by saidtransmission; and means for combining the first, second, and thirdvalues to form said third control signal.
 2. A traction control systemas set forth in claim 1 wherein said means for combining said valuesincludes means for delaying said first value by a variable time intervalprior to combining said values to form said third control signal.
 3. Atraction control system as set forth in claim 2 wherein the means forgenerating a second value proportional to the rate of change of theoperating speed of said engine and indicative of the amount of wheeltorque produced by the inertial movement of said engine is bounded bythe first value delayed by the variable time interval.
 4. A tractioncontrol system as set forth in claim 2 including means for decreasingthe duration of said variable time interval in response to increases inthe operating speed of said engine.
 5. A traction control system as setforth in claim 4 further comprising:means coupled to said drive trainfor generating a fourth value proportional to the rate of change of thedriven wheel speed and indicative of the amount of wheel torque producedby the inertial movement of said drive line; and means for combiningsaid first, second, third and fourth values to form said third controlsignal.
 6. A traction control system as set forth in claim 5 furthercomprising:said drive train including a differential connected to a pairof output shafts for driving a pair of wheels; means coupled to saiddrive train for generating a fifth value proportional to the differencein rotational speed of said output shafts indicative of the amount ofwheel torque produced by losses in said differential; and means forsumming said first, second, third, fourth and fifth values to form saidthird control signal.
 7. A traction control system as set forth in claim1 further comprising a filter means for filtering driveline shuffle modedynamics.
 8. A traction control system as set forth in claim 1 furthercomprising means for calculating a driving surface coefficient offriction and providing the calculated coefficient of friction to thecontrol system when wheel slippage is present or when the calculatedcoefficient of friction is greater than a stored coefficient offriction.
 9. In an electronic engine control module for controlling theoperation of an internal combustion engine connected to a transmissionin a vehicle, a mechanism for producing an output signal valueindicative of the estimated magnitude of torque applied through a drivetrain to a drive wheel of said vehicle by said engine and transmission,said mechanism comprising:means jointly responsive to the speed of saidengine and the rate at which fuel is supplied to said engine forgenerating a first signal value indicative of the magnitude of torqueapplied to said drive wheel due to combustion within said engine; meansresponsive to the rate of change of the speed of said engine forgenerating a second signal value indicative of the magnitude of torqueapplied to said wheels as a result of the inertial movement of saidengine; means responsive to the speed ratio of said transmission forgenerating a third signal indicative of the magnitude of torque appliedto said wheels as a result of said transmission; and means for combiningsaid first signal value, said second signal value and said third signalvalue for producing said output signal value.
 10. A mechanism as setforth in claim 9 further including a variable time delay device fordelaying said first signal value by an interval having a controllabletime duration prior to combining said signal values.
 11. A mechanism asset forth in claim 10 wherein said variable time delay device isresponsive to increases in the operating speed of said engine todecrease said controllable time duration.
 12. A mechanism as set forthin claim 11 further comprising:means coupled to said drive train forgenerating a fourth signal value proportional to the rate of change ofthe driven wheel speed and indicative of the amount of wheel torqueproduced by the inertial movement of said drive train; and means forcombining said first, second, third and fourth values to form said thirdcontrol signal.
 13. A mechanism as set forth in claim 12 furthercomprising:said drive train including a differential connected to a pairof output shafts for driving the wheels; means coupled to said drivetrain for generating a fifth value proportional to the difference inrotational speed of said output shafts indicative of the amount of wheeltorque produced by the losses in said differential; and means forcombining said first, second, third, fourth, and fifth values to formsaid third control signal.
 14. A mechanism as set forth in claim 9further comprising means for calculating a driving surface coefficientof friction and providing the calculated coefficient of friction to thecontrol module when wheel slippage is present or when the calculatedcoefficient of friction is greater than a stored coefficient offriction.
 15. In a control system for use with an internal combustionengine in a vehicle, said engine being coupled through a transmission toapply torque to at least one drive wheel of said vehicle by means of adrive train including a differential, an apparatus for generating asignal value indicative of the amount of torque applied to said drivewheel by said drive train, said apparatus comprising, incombination,means for producing a first signal value indicative of therate at which fuel is supplied to said engine, means responsive to saidfirst signal for generating a second signal value indicative of theamount of torque generated by said engine, means for producing a thirdsignal value indicative of the rate of change of engine speed, meansresponsive to said third signal value for producing a fourth signalvalue indicative of the instantaneous amount of inertial torqueattributable to the acceleration or deceleration of said engine; meansfor producing a fifth signal value indicative of the speed ratio acrossthe transmission, means responsive to said fifth signal value forproducing a sixth signal value indicative of the instantaneous amount ofinertial torque attributable to the acceleration or deceleration of saidtransmission; and means for combining said second, said fourth and saidsixth signal values to produce an output signal having a valueindicative of the instantaneous torque applied to said drive wheel bysaid engine and transmission.
 16. An apparatus as set forth in claim 15further comprising:means for producing a seventh signal value indicativeof a differential speed across the differential; means responsive tosaid seventh signal value for producing an eighth signal valueindicative of the instantaneous amount of inertial torque attributableto the acceleration or deceleration of said drive train; and means forcombining said second, said fourth, said sixth, and said eighth signalvalues to produce an output signal having a value indicative of theinstantaneous torque applied to said drive wheel by said engine,transmission and drive train.
 17. An apparatus as set forth in claim 15wherein said means for combining said second and said fourth signalvalues further comprises means for introducing a relative time delayinterval between said second and said fourth signal values prior toforming said combination.
 18. An apparatus as set forth in claim 17wherein said means for introducing said relative time delay interval isresponsive to the speed of said engine for increasing the duration ofsaid interval in response to decreases in the speed of said engine. 19.An apparatus as set forth in claim 18 wherein said means for producingsaid second signal value comprises processing means for generating saidsecond signal value at times separated by a predetermined duration, andwherein the duration of said relative time delay interval is an integralmultiple of said predetermined duration selected in response to thecurrent speed of the engine.
 20. A traction control system as set forthin claim 19 further comprising means for bounding said second value bysaid first value delayed by the variable time interval and means forfiltering driveline shuffle mode dynamics.